1. Field of the Invention
The present invention relates to improvements in rare gas-halogen excimer lasers, and in particular, to improvements which increase the length of time, reliability and efficiency with which such lasers operate.
2. Background
An excimer laser uses a rare gas such as krypton, xenon, argon or neon, and a halide gas or a gas containing a halide, for example F.sub.2 or HCl, as the active components. The active components and other gases are contained in a pressure vessel provided with laser optics at each end and longitudinally extending lasing electrodes for causing a transverse electrical discharge in the gases. The discharge causes the formation of excited rare gas-halide molecules whose disassociation causes the emission of ultraviolet photons constituting the laser light. The laser gases are circulated between the lasing electrodes by a fan and cooled by a heat exchanger within the pressure vessel.
Excimer lasers emit pulses of ultraviolet light radiation and have potentially many practical applications in medicine, industry and communications. This potential has remained to the most extent unfulfilled because of a number of problems that limit the length of time excimer lasers will operate without substantial maintenance or problems.
One of the problems encountered in efforts to achieve a practical excimer laser is the difficulty of obtaining a homogeneous volumetric discharge between the longitudinally extending lasing electrodes. Inhomogenous arcing between the electrodes causes their eventual destruction as well as contamination of the laser gases and optics with sputtered electrode material.
In order to overcome this difficulty, pre-ionization of the gas volume has been provided. This pre-ionization creates a low level electron cloud prior to the laser-exciting electrical discharge, and results in a homogeneous discharge. One type of pre-ionizer uses a non-solid, perforated, metallic longitudinally extending electrode separated from a co-axial ground electrode by an insulator. The pre-ionizer electrodes are co-axially situated within one of the lasing electrodes, which is made of conductive screen or mesh. The voltage applied to the pre-ionizer electrodes creates a plasma around the pre-ionizer electrodes which produces ultraviolet radiation. The ultraviolet radiation passes through the screen of the surrounding longitudinal lasing electrode to the area between the lasing electrodes and ionizes a portion of the gas there, allowing for a homogeneous discharge when an electric pulse is applied to the lasing electrodes. These additional components within the laser cavity are potential sources of contamination of the laser gases. Contamination of the laser gases during the operation of an excimer laser quenches the laser action.
Contamination of the laser gases or the optics in the pressure vessel requires that major maintenance and/or disassembly of the laser take place. Prior to the present invention, the lifetime of excimer lasers was on the order of a few tens of millions of pulses. It will be readily appreciated that at typical pulse rates between 10 and 500 pulses per second, the operating time between such maintenance procedures or disassembly is on the order of hours, rendering such excimer lasers impractical for many, if not most, applications. In addition, because the toxic and corrosive gases used in excimer lasers must be carefully handled during disassembly of the laser and subsequent reassembly, such procedures are neither simple or nonhazardous.
It is recognized by the present invention that contamination in excimer lasers arises from hydrocarbons, water vapor, fluorocarbons and other organic molecules and impurities and that the sources of such contaminants are many and varied. For example, the use of plastic supports for the longitudinal electrodes or as electrical insulators in the pressure vessel permits hydrocarbons and other molecules therein to contaminate the laser gases. Many parts in present day excimer lasers are either made of Teflon or have a Teflon coating on them or are made of epoxy resins, polyvinyl chloride, or other plastic materials. Teflon is perhaps the best of such materials because it is relatively inert to the corrosive effects of halogens while also being an electrical insulator. However, even Teflon and all the other plastic materials contaminate the excimer laser gases by virtue of the presence of hydrocarbon and/or fluorocarbon molecular structures. In addition to quenching the laser action, fluorocarbon or hydrocarbon molecular structures may be dissociated by ultraviolet radiation emitted from the gas discharge causing carbon or hydrocarbons to be deposited on the laser optics, which eventually destroys the laser output.
Another source of contamination arises from the use of a fan within the pressure vessel to circulate the laser gases. In particular, fluorocarbon grease or other lubricant used in connection with the bearings upon which the shaft of the fan rotates is a source of contaminants. The use of dry bearings for the fan shaft is not satisfactory as dry bearings have not proved to be sufficiently long-lived so as to be practical.
Additional problems arise from water vapor that may be introduced into the pressure vessel. The halide gases in the laser system form inherent metallic halides on the electrode surfaces. Water vapor from the air may gain entry into the pressure vessel as a result of maintenance procedures that take place after the laser gases are contaminated or the optics degraded. The metallic halides in the pressure vessel react with the water vapor to form highly corrosive compositions. For example, nickel fluoride and nickel chloride react with water vapor to form hydrofluoric acid and hydrochloric acid, respectively, which are corrosive substances that seriously degrade the materials used for the optical windows in the pressure vessel, e.g., quartz, calcium fluoride or magnesium fluoride.
Contamination within the pressure vessel, whether it is contamination of the laser gases which tends to quench the laser action or it is contamination of the optical windows on the pressure vessel, results in impractically short operating times between maintenance procedures. These maintenance procedures, which may involve changing the gases in the pressure vessel and cleaning and/or replacing the optical and other components in the pressure vessel are time consuming and costly. During the time that such procedures are taking place, the excimer laser is not available for use. In addition, the halogen gases used in excimer lasers and other gases that might be formed from impurities are highly toxic. When such gases have to be handled, such as during maintenance procedures, the possibility of their escape into the surrounding atmosphere is a safety hazard. The safety hazard is particularly critical if the excimer laser is utilized in medical procedures and is being serviced proximate to where such procedures take place.